Disk-based storage devices such as hard disk drives (HDDs) are used to provide non-volatile data storage in a wide variety of different types of data processing systems. A typical HDD comprises a spindle which holds one or more flat circular storage disks, also referred to as platters. Each storage disk comprises a substrate made from a non-magnetic material, such as aluminum or glass, which is coated with one or more thin layers of magnetic material. In operation, data is read from and written to tracks of the storage disk via a read/write head that is moved precisely across the disk surface by a positioning arm as the disk spins at high speed.
The storage capacity of HDDs continues to increase, and HDDs that can store multiple terabytes (TB) of data are currently available. However, increasing the storage capacity often involves shrinking track dimensions, bit lengths or other features in order to fit more data onto each storage disk, which can lead to a variety of problems, including degraded on-track recording performance, as well as off-track recording performance issues such as adjacent track erasure and far track erasure.
A number of techniques have been developed in an attempt to further increase storage capacity. For example, a technique known as shingled magnetic recording (SMR) attempts to increase storage capacity of an HDD by “shingling” a given track over a previously written adjacent track on a storage disk. In another technique, referred to as bit-patterned media (BPM), high density tracks of magnetic islands are preformed on the surface of the storage disk, and bits of data are written to respective ones of these islands. Other techniques include, for example, heat-assisted magnetic recording (HAMR) and microwave-assisted magnetic recording (MAMR). The HAMR technique utilizes a laser to locally preheat an area on the disk surface prior to recording in that area. In the MAMR technique, an additional write head is configured to emit an AC magnetic field that excites ferromagnetic resonance in the media, building up energy that eases the process of writing data.
HDDs often include a system-on-chip (SOC) to process data from a computer or other processing device into a suitable form to be written to the storage disk, and to transform signal waveforms read back from the storage disk into data for delivery to the computer. The SOC has extensive digital circuitry and has typically utilized advanced complementary metal-oxide-semiconductor (CMOS) technologies to meet cost and performance objectives. The HDD also generally includes a preamplifier that interfaces the SOC to the read/write head used to read data from and write data to the storage disk. As is well known, the read/write head may comprise, for example, separate read and write heads.
The preamplifier generally comprises one or more write drivers that provide corresponding write signals to the write head in order to write data to the storage disk. Such write signals are generally characterized as current signals, but may alternatively be characterized as voltage signals. Data bits are usually each stored as a group of media grains oriented in a common magnetization direction (e.g., up or down). In order to record a given data bit, the write driver generates a write signal that transitions from a negative write current to a positive write current, or vice-versa, where the magnitude of the write current from zero to its peak value may be in the range of about 15 to 65 milliamperes (mA), although different values can be used. For example, higher peak values up to about 165 mA are used in some implementations.
At the completion of a given write operation, the write head may exhibit remanent magnetization after the write current has been turned off. This residual magnetization or “domain lock up” can be the cause of a phenomenon known as erase after write (EAW), where a non-energized (i.e., zero write current) head is seen to erase or degrade previously-written tracks of the disk. These previously-written tracks may comprise user data or even fixed servo sectors that are used to control the tracking of the radial position of the write head. In order to address the EAW problem, a degauss signal may be applied to the write head by the preamplifier immediately after completion of the write operation.
The typical degauss signal waveform includes current pulses that repeat at a fixed frequency and decay in amplitude over time, usually from a write mode current level to a zero current level or other small final value. The degauss signal waveform is therefore an alternating current waveform, with pulse amplitudes decaying at a specified rate. The waveform may include overshoot on each pulse. In some arrangements of this type, the steady state and overshoot portions of the waveform decay at substantially the same rate. Thus, in such arrangements, the ratio between the steady state and overshoot portions is kept substantially constant for the duration of the degauss signal.
It is also possible to provide separate control mechanisms for controlling respective steady state and overshoot portions of the waveform, such that different decay rates can be provided for each of these portions, as described in U.S. patent application Ser. No. 13/447,741, filed Apr. 16, 2012 in the name of B. Livshitz et al. and entitled “Storage Device Having Degauss Circuitry with Separate Control of Degauss Signal Steady State and Overshoot Portions,” which is commonly assigned herewith and incorporated by reference herein.
The frequency of the degauss signal waveform can also be varied. Waveforms of this type are referred to as “chirped” degauss signal waveforms. See U.S. patent application Ser. No. 13/186,445, filed Jul. 19, 2011 in the name of J. S. Goldberg et al. and entitled “Magnetic Storage Device with Chirped Write Head Degaussing Waveform,” which is commonly assigned herewith and incorporated by reference herein.